1. Field of the Invention
The invention relates to the art of electrometallurgy and more particularly to a method of controlling relative movement between an ingot and a mold, as well as a mold for carrying out the method in the process of electroslag melting.
2. Description of the Prior Art
Methods of controlling the relative movement between an ingot and a mold in electroslag melting are presently widely known in the pertinent art, wherein an ingot and a mold are relatively moved by varying their speeds in response to variations in the level of the metal pool. The level of the metal pool is monitored by sensors of the heat, induction, photoelectric, and other types, installed in the mold wall.
Among these is a method of controlling relative movement between an ingot and a mold as disclosed in USSR Inventor's Certificate No. 371,807, Int. Cl..sup.2 C21C 5/56, published Dec. 6, 1972, wherein the level of the metal pool is monitored by the metal temperature.
The temperature is determined by measuring the intensity of the radiation from the ingot surface by a photoelectric sensor installed in the mandrel. In response to variations in the temperature of the ingot surface at the sighting zone of the photoelectric sensor, which variations are indicative of the poor level variations, the relative speed of the ingot and the mold is correspondingly varied so that a predetermined level of the metal pool is restored.
However, instability of the sensor signal, caused by a different in thickness of the slag skin on the inner surface of the hollow ingot varied with the slag used in the melting process, prevails, which leads to an appreciable reduction in the accuracy of monitoring and hence the quality of the ingot produced. Moreover, thermal protection for the photoelectric sensor against high temperature and mechanical damage is required, which results in a rather complicated construction of the mandrel and a lower dependability of the sensor and the control system as a whole. Special-purpose protective filters, which are indispensable to eliminate pickups of the furnace working current in the weak-current output circuits of the photoelectric sensor, also greatly complicate the equipment used to practice the method.
From the above it is to be noted that the prior art method of controlling relative movement between an ingot and a mold is not useful to invariably hold the melt level, and, hence, to produce high-quality ingots because of a low accuracy in pool level control.
The mold, which is controlled in its movement with respect to the ingot as described above (USSR Inventor's Certificate No. 371,807, Int. Cl.sup.2 C21C 5/56), has walls which, in combination with a base plate and mandrel, define a melting space for producing hollow ingots.
Known in the prior art is a mold disclosed in British Pat. No. 1,480,216 published Oct. 5,1973. This mold has walls which, in combination with a base plate, define a melting space for producing solid ingots. An upper mold section is electrically insulated from the ingot and from the lower mold section along the perimeter thereof, disposed above the level of the metal pool and in contact with the bath of molten slag.
The molds of the type disclosed are rather complicated structures to produce and to use since thermal protection is required for electric insulation from the action of the molten slag and a reliable fluid seal is of importance at the junction of the upper and lower mold sections along the inner perimeter thereof.